Miniature mechanical impedance head



73-4532 5R 7 KR; amasmess V Feb. 4, 1969 F. SQHLQSS 3,425,255

MINIATURE MECHANICAL IMPEDANCE HEAD Filed Nov. 30. 1965 V I X 44 PFIE V AMP. v

MIXER DISPLAY L... PRE J AMP.

INVENTOR. FRED SCHLOSS ATTY. I

3,425,265 MINIATURE MECHANICAL IMPEDANCE HEAD Fred Schloss, Arlington, Va; assignor to the United- States of America as represented by the Secretary of the Navy A Filed Nov. 30, 1965, Ser. No. 510,711 US. Cl. 715-671" 7 Claims Int. Cl. G011! 29/00; Gillm 7/00 ABSTRACT OF THE DISCLOSURE A mechanical impedancemeasuring device hhving a pedestal supported acceleration element mounted to respond to inertial bending and a force responsive element also mounted to respond tobendihg of thesupp'ortmerriber attached to the specimen whose impedance is being measured. The two sensors are electrostatically separated and the assembly is capable ofbeing constructed to measure mechanical impedances of the order of oneg'ram or less.

1" he invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to a miniature mechanical im- ""pedance 'de'vicef'Mor'e specifically the device includesan acceleraton sensing element and a force sensing element mounted together for measuring the mechanical impeglanceof very small and light test structures,

Prior impedance measuring devices have had the disadvantages that they were too large to be used with small .models and were likelyto stiffen the model with the result that it was not possible to get realistic measurements from small models using these known impedance measuring devices. Furthermore, to permit the use of the device with thin walled models, it is desirable to construct the device to have as little mass aspossible between the force sensing element and the specimen being measured,

It is an object of this invention to provide an impedance measuring device for use with small structures.

It is the object of this invention to provide an impedance device having a very small mass. 7 Other objects andgnany of .the attendant advantages of'this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description of a preferred embodiment thereof when considered in connection with theaccornpanying drawings wherein:

effect of the attachment of the-impedance device to thespecimen while providing the necessary good contact for the transmission of the force from the source of vibrating. energy or shaker. The shoulder enables flexing or bending (e.g. elastic bending) of the portion 14 to which the force.

measuring transducer is attached. I Referring now to FIG. 2 fora more detailed description of the construction .of. the device of the invention the closure member 13 is constructed to'aSsume theform ofan accelerometerhaving' a disc member 19 supported by a pedestal 21, the pedestal being attached to or car ried by the closure member 13. Closure member 13 con-.

.tains an opening or passage 22-therein through whicha:

lead 23 is attached to a sensing element 24 which may be of a piezoelectric material or of othr'strain responsive material. The lead 23 extends through the opening 22am a bore 25 in closure member 13, bore'25 extending or passing substantially across a diameter of the element 13 to prevent disturbing the symmetry and balance of the device. The piezoelectric element 24 is, in the pre- 7 ferred embodiment of the invention, a thin disc mounted FIG. 1 is a perspective view of the device of this in was her 12and two end-portions 13 and-14,respeetively, pro- 'viding closures. for the opposite-ends Ofthe body-portion 12. Each of the end pieces is supported along its respec- --tive peripheral edge by'body portion 12 and is provided with means for attachment either to the source of vibraance of which is being measured. This attachment means may be a threaded member, as 15 and '16 in FIG, I, or in some cases may bemade with-cement or any other ap-' propriate fastening means. In the preferred embodiment,

prior art wherein compressional force was employed to tion energy -(not shown) or to t'he specimen the impedseparate the accelerometer 13 from the force sensing ele- "nrs counter bore 26 in disc member 19, as by cementing.

The disc member 19 and .pedestalllconstitute .a strain,

filter to preventstrains directly due to the force being' sensed by the accelerometer. The pedestal 21 in this" preferred embodiment is made integral with the elements 13 and 19 but may be made in other ways so long as the connection is sufficiently rigid.

The body member 12 i substantially a hollow cylinder with a webbed portion 28 which serves to electrostatically ment 14. The end pieces 13 and 14 are held to the ends of the body member .12 'in the preferred embodiment by a plurality of small bolts (no reference numerals) pas s ing through a bore in end piece 13 and body member 12 and being threadedly engaged in end piece 14.

Thelorce sensing element 14 is constructed by mounting a piezoelectric element 31 in a recess 32 in the element 14. There is also provided in element 14 a' bore 33 through which a lead 34 is passed to attach it to the piezoelectric elementrdl. This bore, 33 is also constructed along the diameter of the element 14 to avoid distorting the dynamic balance of the device, as was bore 25. Element 31 is designed to match the piezoelectric element 24. This matching of the piezoelectric elements 24 and 31 as to size, weight and response to minimize the effects of transverse forces on the impedance device and to insure that both elements have substantially the same time constants thus to prevent the introduction of substantially any relative phase shift in their outputs'at low frequencies. Said piezoelectric elements 24 and 31 are smaller than'the interior of the cylindrical body member 12 to permit flexing of the crystals 24 and 3 1, with the bending of their respective support members to which they are attached. By flexing or ben d ingis meant'the ,l'Ciili-lt'fijdifif placement of onepor-tion of the member and,- in this case, 'within the elastic 'limits. This is in contrast to the which is enact the objects of the invention; and also pro'-' vides for coaxial alignment of the crystals. Further, by

ness of construction. Magnesium has alsonbeen used for elements 13 and 14. The piezoelectric elements, which are normally of dense materiaLare made as thintisprac tical to reduce the overall weight of the device.

In normal operation ther forcemeasuring element 14 is attached to the model ordevice whose impedance is being measured and the accelerometer element 13 is attached to the source or vibrating energy. The outputs of the accelerometer" and force nie'asuring elements are connected toappropriate amplifying measuring circuits (not shown).

The circuit of FIG. 5 is merely illustrative and Ca1+ sist' of the sensing elements 24 and 31 having one face grounded and the other face connected through amplifying circuits 41 and 42 to ii mixing"circuit-43' and thento a display device 44, allof elements which" are conventional. i

Obviously, many modifications and variations of the, present-invention are possible in the light of the above teachings. It 'is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is: V 1. A mechanical impedance device comprising: a hollow cylindrical body member; 7 first flexible means connected to one end of said body member for attaching said device to a source of vibratory force; second means comprising flexible body meansconnected to the other end of said body member and. oriented for flexibility in the direction of the axis of the body member, for flexing responsive to vibratory force applied to said body member; said second means attaching said device to a specimen under test: first flexible sensing means mounted on and mechani cally coupled to said first flexible means for measur abiy sensing the acceleration produced by the applied force;

second flexible sensing means of smaller diameter than --fiexure with said'second body means, the magnitude said acceleration and force sensing means being axially aligned and impedance matched to have substantiall the same time constants; i whereby the mechanical impedance of the specimen may be determined. 2. A device as claimed in claim 1 in which said first flexible means for attaching the device includes an and closure for said body member of said device.

3; An impedance deviceuas claimed in claim 2 in whichsaid first flexible means for sensing the acceleration comprises: i

a disc member having at counterbore in one face thereof, said counter bore being centrally located with respect to the periphery of said disc;

1 a pedestal member interconnecting said disc and said end closure; v

7 said pedestal member haying a cross sectional area much 5. A device as claimed in claim 2 in which said flexible vi 6. A device as claimed in claim 5 in which said force ensing means comprises:

a counter bore in the center of said second end closure;

and i a piezoelectric sensing element mounted in said counter bore. for flexing with said second end closure in re-' sponse to the applied vibratory force. 7. A device as claimed in claim 1 in which said body member has a conductive web member spaced from each end of the cylinder-for clcctrosta'tically isolating said acceleration sensing means and said force sensing means from each other.

References Cited iii-urea STATES PATENTS 3,070,995 3/19 3 Schlossetal. .73--67.'1 075 099 1/19 3 Shoor 310-8.4

RICHARD C. QUEISSER, Primlrl ylExaminer J, P. BEAUCHAMP, Assistant Examiner. v

Orlacchio 3108.4' 

